Arrhythmias are variations in heart rate from the normal sinus rate range of approximately 60 to 120 beats per minute (bpm) prevalent in healthy adult humans with normally functioning hearts. Bradycardia is characterized by rates below 60 bpm, whereas for tachycardia the rates generally range upwardly from 120 bpm. Typically, tachycardia is experienced as a result of such factors as physical stress (exercise), emotional stress (excitement), consumption of alcoholic or caffeinated beverages, ingestion of certain drugs such as nicotine, and so forth. The heart rate of a healthy person will gradually return to the sinus rate after removal of the tachycardia-inducing factor(s). However, arrhythmias require special medical treatment. For example, fibrillation is a high rate arrhythmia tachyarrhythmia characterized by completely uncoordinated contractions of sections of conductive cardiac tissue of the affected chamber of the heart, resulting in a complete loss of synchronous contraction of the overall mass of tissue. As a consequence, the chamber ceases to pump blood effectively and, in the case of ventricular fibrillation, the lack of oxygenated blood to the tissues will lead to death within minutes.
Implantation of a cardiac pacemaker has been a typical procedure of choice for treatment of bradycardia patients. The pacemaker pulse generator is implanted in a pouch beneath the skin in the patient's chest and delivers electrical impulses to electrodes positioned at the patient's heart via one or more catheter leads, to stimulate the heart to beat at a desired rate in the normal sinus range. Cardiac pacing has also found increasing usage in the management of tachyarrhythmia. In essence, the heart may be artificially stimulated at a faster than normal pacing rate to terminate a tachycardia or to suppress premature atrial or ventricular contractions which could otherwise lead to supra-ventricular or ventricular tachycardia, flutter, or fibrillation. The pulses delivered to the heart for pacing therapy need only be of sufficient magnitude to stimulate the excitable myocardial tissue in the immediate vicinity of the pacing electrode.
More recently, the automatic defibrillator has been proposed for implantation in cardiac patients prone to suffer ventricular tachycardia and/or fibrillation. The device is adapted to shock the heart with electrical pulses of considerably higher energy content than is delivered in pacing pulses. Upon detecting fibrillation, one or more high energy "counter-shocks" are applied to the heart to overwhelm the chaotic contractions of individual tissue sections and re-establish organized spreading of action potential from cell to cell of the myocardium, thereby restoring the synchronized contraction of the mass of tissue. In general, prior art implantable defibrillators have been implemented to detect fibrillation from the patient's electrogram waveform and/or the absence of a "mechanical" function such as rhythmic contractions or pulsatile arterial pressure, and, in response, to deliver a fixed therapy typically consisting of one or more shocking pulses of preset waveform and energy content.
For example, detection of fibrillation activity by monitoring the electrogram signal has been achieved with conventional sense amplifier configurations involving fixed sense margin, fixed or selectively adjusted gain and fixed bandpass characteristics. This type of detection tends to neglect the characteristic differences between QRS complexes and fibrillation waveforms. Fibrillation waveforms are characterized by erratic amplitude modulated sinusoids of relatively low frequency in a narrow band (3-12 Hz). On the other hand, QRS complexes have relatively constant amplitude, sharp peaks with significant high frequency content and thus a broader frequency spectrum.
The aforementioned co-pending U.S. patent application Ser. No. 875,218 discloses an improved medical device for treating ventricular tachycardia and defibrillation, which uses sophisticated techniques for detecting and distinguishing the arrhythmia from normal high rates, and a hierarchical approach to the aggressiveness and delivery of therapies to terminate the arrhythmia. The functions of bradycardia and anti-tachycardia pacing-type therapies, and cardioversion and debifrillation shock-type therapies, are integrated to provide a coordinated approach to the management and treatment of ventricular arrhythmias. A significant aspect of that approach is to provide flexible sequencing among the therapies, with appropriate regard to hemodynamic tolerance (or intolerance) of the patient to the detected arrhythmia, and sophisticated detection of arrhythmias together with means for distinguishing those episodes for which treatment is required (such as re-entrant tachycardias) from those which are not associated with cardiac or other disease (such as exercise-generated sinus tachycardias). A multiplicity of hierarchical detection algorithms and therapeutic modalities are selectively available to the physician to detect and treat classes of ventricular tachycardia according to their respective positions in the heart rate continuum, and thus according to hemodynamic tolerance or intolerance of the patient to the tachycardia, with backup capabilities of defibrillation and bradycardia pacing for cardiac arrhythmias, including postshock bradycardia.
In an embodiment described in the 875,218 application, the cardiac stimulator permits selective partitioning of the heart rate continuum into a plurality of contiguous tachycardia classes of progressively higher rate ranges, the lowest and highest of these classes being bounded respectively by regions of the continuum denoting sinus rate and fibrillation. Each of the rate ranges and the latter regions may be programmed by the physician, as may be necessary to meet the particular needs of the patient's disorder and the flexibility of the therapy regimens to be prescribed. The hierarchical detection system employed in the stimulator for detecting cardiac episodes indicative of arrhythmia and for distinguishing between normal and abnormal tachycardias among the detected episodes uses criteria of greater or lesser stringency depending on the location of the episode in the rate continuum. The device permits programming of the various detection criteria for tachycardias and fibrillation, and includes plural combinations of basic detection criteria in each category. The arrhythmia detection system disclosed in that application also utilized signal processing including amplification with automatic gain control and bandpass filtering according to the present invention, for enhancing sensitivity to the differences between the electrical and physical characteristics of sinus rhythms and arrhythmia waveforms.
It should be noted that the prior art has suggested the analysis of waveform morphology for fibrillation detection. Moreover, it is known to use automatic gain control in defibrillation systems. A principal object of the present invention, however, is the provision of a system and method for arrhythmia detection which recognizes and takes advantage of the characteristic differences between sinus rhythm and arrhythmias, including those mentioned above, utilizing automatic gain control and bandpass filtering in a feedback loop.
Another significant object of the present invention is to provide an implantable cardiac stimulator utilizing automatic gain control in interaction with arrhythmia detection, as well as with special bandpass characteristics and bradycardia pacing.
A further important object of the present invention is to provide an approach for dealing with interactions between gain and pacing in an implantable device capable of treating bradyarrhythmias as well as tachyrhythmias.
Yet another object of the invention is to provide an implantable device which implements the foregoing objectives in treating tachycardia and fibrillation, and which is adapted also to treat post-shock bradycardia.
Still another object of the present invention is to provide a cardiac arrhythmia detection system and method employing automatic gain control in conjunction with bandpass filtering in a feedback loop, wherein the margin of sensitivity to the various waveforms encountered varies to enhance the capability and speed of detection of the arrhythmias of interest, such that an appropriate therapy is quickly applied to return the heart to normal sinus rhythm.